The invention relates to a process for producing inductively operating counting systems, for example inductively operating phone cards.
The normal phone cards that are currently used may be divided into two different categories. Those used in Europe and the USA employ magnetic layers for information storage. The phones in which the card is to be used accordingly include a device for reading and writing information on the card. Thus, stored information, for example the number of charge units still left, is read and overwritten with new information at specific time intervals.
In a further type of phone cards, as are used for example in Brazil, another, substantially simpler principle of information storage is used. These phone cards consist of a dielectric medium on which a thin structured metallic layer has been applied. The essential feature of this metallic layer is its geometrical shave, which is characterised by a large number of fine, conductor-like webs or ridges. The card functions in the following way: the telephone in which the card is used induces successive induction currents in the card which cause these fine webs or ridges to melt one after another, thereby using up the units on the card. These very fine ridges thus function like a fuse. When all these ridges have melted, the card is used up. Such phone cards and the associated phones are simple in design and construction, inexpensive and robust, since no magnetic writing or reading head is required. Phone cards of this type are hereinafter termed inductively operating phone cards. Naturally, this principle can also be used as a counting system for other units.
According to Brazilian Patent Application 9105585 A, these cards are produced by a process in which a first, conducting layer having a relatively high resistance is first of all applied by chemical means on an impermeable, non-porous substrate. A substantially thicker layer of a metal or metal alloy having a considerably lower resistance than that of the first layer is then applied by electrolytic deposition. The second layer consists for example of a tin-lead alloy, while the first layer consists of a currentless deposited nickel layer.
Plastic substrates, preferably of acrylonitrile-butadiene-styrene (ABS) polymers, polyvinyl chlorides or epoxy resins are employed as starting materials in the production process that is currently used. For example, two films or sheets of the aforementioned materials having the appropriate dimensions, e.g. 400.times.600 mm, are first of all welded together. A so-called "multicopy" is thereby obtained. A preliminary treatment is then performed by etching, brushing or similar processes to roughen and/or hydrophilise the nonconducting surface, so as to ensure a sufficient degree of adhesion of the metallic layer to be applied without using an external current.
This process step is then followed by a catalytic activation, for example by coating the plastic surface with palladium nuclei. A chemically reductive metallisation of the whole surface is then carried out, in practice generally a currentless nickel-plating. These process steps are known per se from the metallisation of plastics.
The next step is the formation of a conductive pattern on the metallic layer, especially the fine metal ridges. For this purpose, in a similar way to the production of printed circuit boards, a photo-structurable layer is applied, for example by laminating a photoresist film. So-called negatively operating dry films are used for this purpose, which are also conventionally known in printed circuit board technology. The illumination and development of the conductive pattern are carried out by conventional methods, i.e. after developing, the resist is removed from those places where the conductive pattern is subsequently to be formed. Finally, the metallic layer applied by metallisation without the use of external current is galvanically reinforced at the bare or exposed sites, i.e. in the resist channel, and in fact preferably with a tin-lead alloy. In order to produce the actual conductive pattern the photoresist film is first of all removed, as a rule by treatment with aqueous alkaline solutions (resist strippers). The currentless deposited metallic layer that is now exposed then has to be removed by etching. The subsequent coating with a suitable varnish or lacquer protects the card, and the protective lacquer may also receive a decorative pattern.
The sheets welded together at their edges are then normally separated from one another and the individual cards are formed by punching or cutting.
If desired the card can be divided into two parts. The disadvantages of the existing process, which is mainly aimed at the so-called subtractive process normally used for printed circuit boards, are firstly the difficulty in reproduciblv etching the melt ridges, which are only about 100 .mu.m wide, with a sufficiently high degree of precision. The high demand on precision is a result of the function of the ridges, which of course must have a defined electrical resistance, which in turn determines the melt behaviour (W=I.sub.2 .times.R). It is found in practice that the reject rate is considerable on account of the problems involved in etching the ridges.
The reasons for this are on the one hand the resolution of the dry film resist, and on the other hand the technical limits in etching the conduction part.